1. Field of the Invention
The present invention relates to a method for manufacturing a quantum cascade laser.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2001-320136 (hereinafter referred to as Patent Reference 1) discloses a quantum cascade laser device. FIG. 12 is a perspective view of the quantum cascade laser device. The quantum cascade laser device is represented by reference numeral 100 and includes a lower cladding layer 101, an active region 102 disposed on the lower cladding layer 101, and an upper cladding layer 103 disposed on the active region 102. The active region 102 and the upper cladding layer 103 form a semiconductor mesa structure having a stripe shape extending in a predetermined direction. Side surfaces of the semiconductor mesa structure form a mesa shape. Dielectric layers 104 are disposed on the semiconductor mesa structure. An opening is formed in the dielectric layer 104 at a top portion of the semiconductor mesa structure. An electrode 105 is disposed over the dielectric layers 104 and formed in the opening. Therefore, the electrode 105 is directly in contact with the top portion of the semiconductor mesa structure through the opening. An electrode 106 is disposed on the back surface of the lower cladding layer 101.
An optical semiconductor device, as well as the quantum cascade laser device disclosed in Patent Reference 1, has a semiconductor mesa structure and includes an electrode extending from a top portion of the semiconductor mesa structure to regions outside the semiconductor mesa structure. The semiconductor mesa structure of the optical semiconductor device has an uneven surface having a convex shape. In the case of forming an electrode on a wide region including the semiconductor mesa structure, the electrode is broken at a stepped portion between the top portion and a side surface of the semiconductor mesa structure or has a thin portion on the stepped portion or another failure arises.
In particular, the larger the inclination of a side surface of the semiconductor mesa structure is, the more difficult the formation of a metal layer on the side surface thereof is. For example, when the side surface of the semiconductor mesa structure is perpendicular to a surface of a substrate, it is difficult to form the metal layer on the side surface of the semiconductor mesa structure. This leads to the breakage of the metal layer or the reduction in heat dissipation performance thereof. However, the less the inclination of the side surface of the semiconductor mesa structure is, the larger the width of the semiconductor mesa structure is. Thus, in this configuration, it is difficult to effectively narrow the current injected into the semiconductor mesa structure. Therefore, forming a good metal layer used to form an electrode over the top portion and side surface of the semiconductor mesa structure conflicts with effectively narrowing the current injected into the semiconductor mesa structure in terms of the inclination of the side surface of the semiconductor mesa structure.